The present invention relates to coal fed power plants, and pertains particularly to an improved control system for a power plant having pressurized combustion or gasification of coal for powering a gas turbine to generate electric power.
There always exists in the power generating industry a need for more efficient power plants for converting fossil fuels to electrical power. The most efficient commercially available technology for power generation systems is the combined cycle gas/steam turbine operating on premium fuels. However, the cost and uncertainty the availability of premium fuels make premium fuel combined cycle power generating plants impractical. Therefore, there exists an even greater need for power plants capable of efficiently handling lower grades of fuels, particularly solid fuels, such as coal, that exist in abundance in many regions of the world for powering gas turbines.
Gas turbines can be operated from either pressurized combustion or gasification of coal or a combination of the the two. Thus, the pressurized combustion or gasification of coal allows the integration of a gas turbine for generating electrical power. One major problem of these systems is the safe shutdown of the turbine in case of malfunction of the turbine, such as a loss of load or balance of the turbine.
The prior art approach is to use a fast acting valve to shutoff or partially shutoff the flow of hot gases or fuel to the turbine, similar to shutdown systems used in natural gas fired combined cycle plants. The purpose of this fast acting valve is to allow rapid shutoff of the gas flow from the pressurized gasifier and/or combustor (PG/C) to the expander inlet in the event of loss of load on the gas turbine's generator or other malfunction. A major problem with this approach is that a complex and costly fast acting, high temperature valve must be used to allow an emergency shutdown of the system. The valve must operate under very high temperature, must close very quickly (on the order of less than 0.3-1.0 seconds), and must be very effective and reliable. In order for the valve to be effectively rapid or fast acting, a butterfly valve is typically used. This valve technology is generally considered to have a high degree of risk. Such fast acting butterfly valves cannot typically provide a tight seal, so some of the gas will leak through the valve when it is closed. This can pose two problems:
1. If too much hot gas leaks through the valve during a shutdown, the gas turbine may continue to accelerate, resulting in damage to the turbine and a potential safety hazard. PA1 2. During a start-up of the plant, the first step is to start-up the gas turbine, firing oil or natural gas in a start-up burner. This occurs before any of the components in the PG/C have been exposed to hot gases. If the subject valve does not provide a tight seal, hot gases from the gas turbine's start-up burner can leak back into the PG/C and could cause damage to components that have not been preheated.
Thus, the start-up and shutdown of such an integrated coal fired combined cycle facility could be greatly simplified by having a valve arrangement and an approach which uses a valve which provides a tight seal, but is not required to be fast acting.
An example of the traditional prior art approach to the problem of shutdown is disclosed in U.S. Pat. No. 4,744,212 wherein the turbine is shutdown first by valves in the hot gas conduit between the bed vessel of the plant and the turbine. This patent also points up the problem of hot gas flow to the turbine being leakage flow through the valve in the hot gas conduit.
Another example of the prior art is disclosed in Kreij U.S. Pat. No. 4,498,285 and Brannstrom et al U.S. Pat. No. 4,498,286, which are companion cases issued Feb. 12, 1985. The Kreij patent is directed primarily to a special valving arrangement of a blow off valve for rapidly releasing the pressure in the combustion chamber when there is a sudden load drop on the turbine.
The Brannstrom et al patent discloses a valve arrangement and method for short-circuiting the fluidized bed so that it collapses. This stops the combustion in the fluidized bed. The problem of hot gas to the turbine is also pointed out by Brannstrom et al in their patent at column 2, lines 26-33.
The problem is achieving a rapid and complete shutdown of the turbine to prevent over speed in the case of loss of load or other problems. In the case of a loss of load, a turbine can accelerate to dangerous over speed conditions in about 0.3-2.0 seconds. Also, a partial leakage of hot gases to the turbine in the case of a loss of load on the turbine can result in the turbine continuing to accelerate to dangerous speed levels. Thus, an effective shutoff or interruption of the hot gas to the turbine must be achieved rapidly.
Accordingly, an improved power plant to overcome these problems of the prior art would be desirable.